The present invention relates to semiconductor laser devices using an InGaAlN compound semiconductor and a method of fabricating such a semiconductor laser device, and more particularly to a contact layer for such a semiconductor laser device.
InGaAlN compound semiconductors are wide gap semiconductors and have a direct transition type band structure. Therefore InGaAlN compound semiconductors are expected to be applied to light-emitting devices having a radiative wavelength range of from yellow to ultraviolet. In such field of application, light-emitting diodes of double heterostructure have already been put into practical use and, in addition, semiconductor laser devices are being positively developed. The following describes a typical process for fabricating a conventional semiconductor laser device which is shown in FIG. 2. First, an AlN buffer layer 220 of about 0.1 .mu.m is grown on a 6H--SiC substrate 210 at a substrate temperature of about 1050.degree. C. by an metal organic chemical vapor deposition (hereinafter referred to as MOCVD) method. Subsequently an n type GaN layer 230 of 0.5 .mu.m and an n type AlGaN cladding layer 241 of 1.0 .mu.m are grown at the substrate temperature of about 1050.degree. C. Thereafter, the temperature of the substrate is lowered to about 800.degree. C. so that an InGaN active layer of 0.01 .mu.m is grown. Then, the temperature of the substrate is again raised to about 1050.degree. C. so that a Mg doped AlGaN cladding layer 242 of 1.0 .mu.m and a Mg doped GaN contact layer 260 of about 0.5 .mu.m are grown.
After growth of the foregoing layers, the thus formed wafer is removed from the MOCVD apparatus. Then, the wafer is thermally treated at a temperature of about 600.degree. C. in a nitrogen atmosphere for about 30 minutes so that the Mg doped AlGaN cladding layer 242 and Mg doped GaN contact layer 260 are given low resistance p type characteristics. Further, in order to provide an optical waveguide, a dielectric film 270 having an opening of about 1 .mu.m in width is formed on the GaN contact layer 260, and then a p-side electrode 281 is formed over the entire surface of the dielectric film 270 having the opening, and an n-side electrode 282 is formed on the back of the n-type 6H--SiC substrate 210 by using a conventional vacuum evaporation technique. Further, a facet reflector mirror is formed by dry etching according to the reactive ion beam etching (RIBE) technique, and then the wafer is divided into separate elements by a dicing process. In this way, the semiconductor laser device shown in FIG. 2 is completed.
In the above described conventional semiconductor laser device, the electrode 281 is formed on the GaN layer. Forming an electrode on a GaN layer in such a way is disclosed in, for example, Japanese Patent Application Laid-Open No 6-268259. However, the GaN layer has conventionally been unable to provide a high-concentration distribution of p-type carriers and thus it involves a high element resistance. That is, a high drive voltage is involved and this may adversely affect the device reliability.